Browsing by Subject "Perovskite"

Metal-halide perovskites (MHP) comprise a diverse family of crystalline materials whose optoelectronic properties have gathered significant interest recently. Their use in transistors, solar cells, light emitting diodes, and many other applications with significant real-world impacts has been enabled by synthesis techniques that can deposit high quality MHP thin films. The simple yet powerful chemistry involved in solution-processing techniques has allowed for MHP thin films to be deposited in a variety of different ways like spin-coating, inkjet printing, and doctor blading. However, solvent in these techniques can preclude the creation of advanced MHP structures like graded composition films or all-MHP heterojunctions. Additionally, the poor solubility of complex organic moieties in the polar solvents used in solution-processing of MHP materials could prevent the creation of MHP materials with unique photophysical properties. The development of vapor-processing techniques which circumvent the use of solvent to vaporize MHP precursors and deposit thin films has shown promise in addressing these concerns with solution-processing. However, the use of highly energetic precursor vaporization mechanisms has itself raised worries about its broad applicability.

In this dissertation, the deposition of MHP thin films using Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE) is developed as an alternative to current MHP thin film deposition techniques. The ‘dry’ deposition of materials under dynamic vacuum and the use of a low energy infrared laser directly address shortcomings of solution- and vapor-processing techniques, respectively. Using the understanding of RIR-MAPLE developed by previous studies, a double solvent approach was first developed to solubilize and deposit MHP precursors in a manner which maintained the integrity of the resulting films. The viability of this baseline approach was confirmed through the creation of MHP solar cells with competitive performance and thin films of MHP materials with complex organic moieties that demonstrate unique photophysical properties. Subsequent studies of nuanced aspects in RIR-MAPLE deposition of MHP thin films helped develop an understanding of the process-structure-property relationships in play during RIR-MAPLE deposition and in post-processing of the resulting MHP thin films.

Following these baseline studies, unique precursor delivery schemes were developed to demonstrate the versatility of RIR-MAPLE. These schemes were shown to reliably deposit continuous films of MHP materials despite differences in the state of precursors during deposition and crystallization. Finally, a comprehensive study of MHP film formation mechanisms during RIR-MAPLE deposition was undertaken. These experiments categorically described the wetting, nucleation, diffusion, and accumulation essential to MHP film development during RIR-MAPLE deposition. Overall, this work demonstrates some of the most promising aspects of the RIR-MAPLE deposition technique and develops the candidacy of RIR-MAPLE as an MHP thin film technique uniquely positioned to address the shortcomings of other currently established methods.

The power conversion efficiency of perovskite solar cells has increased significantly in the past 10 years from around 13% to over 25%. However, the most common perovskite absorber materials, three-dimensional (3D) perovskites, are challenged by moisture stability which hinders their more widespread commercialization. One approach to increase moisture stability is to incorporate a layer of hydrophobic ligands on top of the absorber layer in the form of a two-dimensional (2D) perovskites, thereby forming a 2D-on-3D heterostructure, however there are significant processing challenges. This dissertation conducts a careful investigation of n = 1 2D perovskite thin films deposited using a technique called resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) to better understand and then improve the heterostructure. The major conclusions of this work are that any halide mixing likely occurs in the 3D layer only at the heterostructure interface due to the site preferences that bromine and iodine have in the octahedra. Several supplemental processing parameters – deposition scheme, growth temperature, and solvent ratio (DMSO:MEG) – were used to successfully increase the average grain size, increase the amount of vertically oriented grains, modify the morphology, and decrease the Stokes shift in (PEA)2PbI4 thin films. Ultimately, out-of-plane conductivity in (PEA)2PbI4 thin films was successfully improved using the sequential deposition scheme, elevated growth temperature, and decreased amount of matrix solvent. The structural improvements and improved out-of-plane conductivity were also demonstrated for the heterostructure when modified processing conditions were used to deposit the 2D layer. The process-structure-property relationships investigated in this work serve as guidelines for tailoring 2D-on-3D heterostructures.

Halide semiconductors have recently emerged as a class of materials that unite outstanding optoelectronic properties with the ability to process device-quality thin films at low or even room temperature. Successful adoption of halide semiconductor-based technologies will, however, be contingent on the development of device architectures and film processing approaches that enable efficient, low-cost devices with stable performance and rigorous study of these materials’ photophysical properties. Herein, the goals are twofold: first, to develop low-cost device processing methods that deliver efficient solar cell performance while managing sources of instability; second, to extend existing thin film processing techniques to novel materials, enabling investigation of their optoelectronic properties.

After general introduction to halide perovskite materials, films and devices in Chapters 1 and 2, Chapter 3 confronts the first device challenge—i.e., reducing solar cell cost—by investigating cheap electron and hole transport layers (ETL and HTL) for halide perovskite solar cells. Efficient CH3NH3PbI3 perovskite solar cells are constructed using earth-abundant ETL CdS and HTL CuCrO2 that are deposited at low temperature (<100 °C). Although CuCrO2 appears to yield an inert interface with CH3NH3PbI3, X-ray photoelectron spectroscopy reveals that CdS can easily release Cd into the CH3NH3PbI3 film. X-ray diffraction (XRD) measurements show that excessive amounts of Cd cause phase segregation of insulating compounds in the perovskite, compromising solar cell performance. Nevertheless, careful optimization of device fabrication avoids this detrimental interaction, leading to solar cells with power conversion efficiency of over 15%. In addition to demonstrating efficient devices using low-cost materials, this work emphasizes the importance of managing interfacial as well as bulk stability.

Chapter 4 focuses on the second device challenge—i.e., managing instability—by developing inherently robust architectures via lamination and hot pressing. This technique circumvents the intrinsic thermal instability of perovskite thin films during processing and forms a self-encapsulating device architecture. Annealing MAPbI3 films under pressure in a specially-constructed tool allows significant grain growth at temperatures that would ordinarily decompose them rapidly to PbI2. However, these temperatures can also activate unexpected reactions with carrier transport materials previously thought to be inert, such as nickel oxide. Applying this knowledge, techniques are developed that avoid reactivity-related problems and recover the targeted solar cell performance.

Chapters 5 and 6 of this dissertation focus on developing deposition methods for new halide semiconductor films, with emphasis in this case on exploration of fundamental physical properties rather than device fabrication. In Chapter 5, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is first used to deposit films of the archetypal halide perovskite CH3NH3PbI3, which possesses properties comparable to those prepared by more conventional methods such as spin coating, as determined by XRD, electron microscopy and optical spectroscopy. CH3NH3PbI3 solar cells fabricated using RIR-MAPLE reach power conversion efficiency of over 12%. RIR-MAPLE is then extended to the deposition of layered lead halide perovskite films incorporating oligothiophene-derived molecular cations, which cannot be controllably deposited by other methods. By varying the number of rings in the thiophene chain and the halide component of the inorganic layers, the photoluminescence emission from these films can be tuned to originate from either the inorganic or the organic component or be quenched altogether, supporting prior computational predictions of the tunable quantum well nature of these types of perovskite structures. Carrier transfer between the inorganic and organic moieties can synergistically populate triplet states in the organic, showcasing the unique physical properties attainable in complex-organic perovskites.

Chapter 6 focuses on the halide semiconductor indium(I) iodide, which possesses elements of its electronic structure like those of halide perovskites, that are often invoked as an explanation for these materials’ remarkable defect tolerance. Indium(I) iodide is prepared in thin-film form by thermal evaporation. A photovoltaic effect is demonstrated for the first time in this material, with solar cells demonstrating ~0.4% power conversion efficiency. Overall, the results advance our scientific understanding of halide semiconductors, and provide crucial pathways by which they can be made more technologically effective, and be studied in greater depth.